Conventionally, heat detectors which sense an occurrence of a fire by sensing heat generated by the fire are proposed. Such heat detectors are roughly classified into differential heat detectors and fixed-temperature heat detectors based on the principle employed for sensing. Such a heat detector, in general, includes a heat detecting unit that senses heat in a monitoring area and a sensing-device main body that raises an alarm according to a state sensed by the heat detecting unit.
The heat detecting unit has a sensor unit which senses heat in the monitoring area and converts a sensed state into another state change. The sensor unit is formed of, for example, a diaphragm which changes shape according to an expansion of air caused by temperature rise, a thermistor which changes resistance according to the temperature, or a bimetal which changes shape in a predetermined direction according to the temperature.
FIG. 20 is a vertical sectional view of a conventional heat detector. A heat detector 200 includes a heat detecting unit 201 and a sensing-device main body 202, and is installed onto an attachment surface such as a surface C of a ceiling. The heat detecting unit 201 is directly connected to the inside of the sensing-device main body 202 by adhesive 203 (see for example Japanese Patent Application Laid-Open No. 2003-196760).